Field of the Invention
Embodiments of the present invention relate generally to a pattern inspection apparatus and a pattern inspection method. More specifically, embodiments of the present invention relate, for example, to a pattern inspection technique for inspecting pattern defects of an object serving as a target workpiece or “sample” used in manufacturing semiconductor devices, and to an inspection apparatus for inspecting a photomask used in manufacturing semiconductor elements or liquid crystal displays (LCDs), and a method thereof.
Description of Related Art
In recent years, with the advance of high integration and large capacity of large-scale integration (LSI) circuits, the line width (critical dimension) required for circuits of semiconductor elements is becoming progressively narrower. Such semiconductor elements are manufactured by circuit formation of exposing and transferring a pattern onto a wafer by means of a reduced projection exposure apparatus known as a stepper while using an original or “master” pattern (also called a mask or a reticle, hereinafter generically referred to as a mask) with a circuit pattern formed thereon. Then, in fabricating a mask used for transfer printing such a fine circuit pattern onto a wafer, a pattern writing apparatus capable of writing or “drawing” fine circuit patterns by using electron beams needs to be employed. Pattern circuits may be written directly on the wafer by the pattern writing apparatus. Also, a laser beam writing apparatus that uses laser beams in place of electron beams for writing a pattern is under development.
Since LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as typified by a 1-gigabit DRAM (Dynamic Random Access Memory), the scale of a pattern configuring an LSI has changed from on the order of submicrons to nanometers. One of major factors that decrease the yield of the LSI manufacturing is due to pattern defects on the mask used, in the photolithography technology, for exposing and transfer printing an ultrafine pattern onto a semiconductor wafer. In recent years, with miniaturization of dimensions of LSI patterns formed on a semiconductor wafer, dimension to be detected as a pattern defect has become extremely small. Therefore, a pattern inspection apparatus for inspecting defects on a transfer mask used in manufacturing LSI needs to be highly accurate.
As an inspection method, there is known a method of comparing an optical image obtained by imaging a pattern formed on a target object or “sample” such as a lithography mask at a predetermined magnification by using a magnification optical system with design data or an optical image obtained by imaging an identical pattern on the target object. For example, the following is known as pattern inspection methods: the “die-to-die inspection” method that compares data of optical images of identical patterns at different positions on the same mask; and the “die-to-database inspection” method that inputs, into an inspection apparatus, writing data (design pattern data) generated by converting pattern-designed CAD data to a writing apparatus specific format to be input to the writing apparatus when a pattern is written on the mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (serving as measurement data) obtained by imaging the pattern. In such inspection methods for use in the inspection apparatus, a target object is placed on the stage so that a light flux may scan the target object as the stage moves in order to perform an inspection. Specifically, the target object is irradiated with a light flux from the light source through the illumination optical system. Light transmitted through the target object or reflected therefrom forms an image on a sensor through the optical system. The image captured by the sensor is transmitted as measurement data to the comparison circuit. After performing position adjustment of images, the comparison circuit compares measurement data with reference data in accordance with an appropriate algorithm, and determines that there exists a pattern defect if the compared data are not identical.
Since the product cycle of semiconductor products is generally short, reducing the time required to manufacture them is an important point. If a mask pattern having a defect is exposed and transferred to a wafer, semiconductor devices made using the wafer become defective. Therefore, it is essential to inspect for pattern defects on the mask. Defects found in the inspection are corrected by a defect correction apparatus. However, correcting all the found defects results in increasing the required manufacturing time, thereby decreasing the product value. With the development of the inspection apparatus, it determines that there is a pattern defect even when a very small deviation occurs. However, when actually transfer-printing a mask pattern onto a wafer by an exposure apparatus, as long as no circuit disconnection and/or no short circuit occurs on the wafer due to such a pattern defect, the circuit can be used as an integrated circuit. Therefore, it is desired to use such a mask in order to perform exposure and transfer, with leaving the defective state without correcting such a defect each time. However, in the inspection apparatus, since the optical system is configured so that a minute deviation can be detected, the mask resolution is set to be higher than that of the exposure apparatus. Therefore, it is difficult for the inspection apparatus to reproduce a pattern image which is to be transfer printed by the exposure apparatus. Thus, it is difficult for the inspection apparatus to grasp how defects found by the inspection apparatus are to be exposed by the exposure apparatus.
Although there is disclosed a dedicated device which inspects an image to be exposed and transferred by an exposure apparatus (for example, refer to Japanese Patent Application Laid-open No. 2001-235853), the device cannot inspect an extremely small deviation. Therefore, it is difficult to inspect a minute deviation by using the device.